AdditiveRunAsHighest requests that the application receives the RunAsHighest flag if and only if nobody else has requested a higher level of elevation.

That means we will overrule a manifest if that manifest is asInvoker, but we will not overrule a manifest if that manifest is requireAdministrator. It also means we will overrule a layer if that layer requests RunAsInvoker, but we will not overrule a layer if that layer requests RunAsAdministrator.

In short, it means that this flag will only be used to increase your level of elevation (to highestAvailable) and will never be used to decrease it (from requireAdministrator).

The reason why this flag exists? Setups. If we think you are a setup, then the setup detection logic applies the VistaSetup layer, which has (as you might imagine) RunAsAdministrator. We don’t want setups to fail for every standard user out there – they still need to prompt. So, this layer will not cause standard users to stop seeing prompts for setups (or manifested apps for that matter). If we had used the existing RunAsHighest flag, then it would have broken setup detection for standard users – and we love our standard users.